Grey iron treatment



March 31, 1970 J. K. MUREK GREY IRON TREATMENT Filed Jan. 27. 1966 INVENTOR. JOSEF K. MUREK ATTORNEY United States Patent Oil 3,503,247 Patented Mar. 31, 1970 :"ice

3,503,247 GREY IRON TREATMENT Josef K. Murek, 4454 Casitas St., San Diego, Calif. 92107 Filed Jan. 27, 1966, Ser. No. 523,457 Int. Cl. B2lj 5/00 US. Cl. 72-377 8 Claims ABSTRACT OF THE DISCLOSURE Grey iron elements, heated in an inert atmosphere to between 1650 F. and 1750 F., are formed in closed, vented, dies at a pressure of from 30,000 p.s.i. to 90,000 p.s.i. applied for on the order of 5 milliseconds, thereby increasing the density and tensile strength of the grey iron by at least A2 percent and 1,000 p.s.i. respectively.

This invention concerns a method for forming shaped articles from grey iron, and the articles obtained therefrom.

Grey iron is unique among alloys because it contains a solid solution of carbon in iron, interrupted by fissures containing flakes of graphite. These flakes act as large dislocations, thereby decreasing the materials ductility. The size and the shape of the graphite flakes are governed by the casting cross-sectional area and the cooling rate. Consequently, the ductility or elongation of grey iron is a function of the size and shape of the graphite flakes and varies inversely with the casting cross-section. The presence of these graphite flakes acting as fissures in reducing ductility and tensile strength has precluded the use of conventional forging techniques for producing parts directly from grey iron.

Grey iron is directly obtained by remelting pig iron and scrap steel. No refining or conversion processes are required. Grey iron is essentially an alloy of the elements iron, carbon, silicon, and manganese which is cast into a mold to determine its final shape. Although most of these same elements are also contained in steels, the high percentage of carbon in grey iron causes the internal formation of a graphite flake. This flake is the cause for the low ductility of grey iron. A description of the metallography of grey iron is found in Kirk-Othmar, Encyclopaedia of Chemical Technology, vol. 8, pages 9234, the Interscience Encyclopaedia, Inc., New York, 1952. Because of the limited amount of processing required to obtain grey iron, the metal is very economical and attractive as an engineering material if it can be directly used in the cast condition.

This invention is directed toward improving the attractiveness of grey iron by changing the orientation of the graphite flake content to the contour of the part and thereby upgrading the mechanical properties. Briefly, the method of this invention for shaping the grey iron element into a desired configuration having enhanced mechanical properties comprises compacting the billet in a closed vented die at a pressure from 30,000 to 90,000 p.s.i. and a temperature from 1650 F. for a short period of time, e.g., of the order of 5 milliseconds. The method achieves a novel shaped object of grey iron having a density increased by at least /2% with an increase in tensile strength of at least 1000 p.s.i. over that of the grey iron as cast, and having at least a layer of substantially laminar oriented graphite particles.

The process and structure discussed above will be more fully understood from the more detailed description of the invention below, referring to the drawings in which:

FIG. 1 is a photomicrograph of a cross-section of the untreated grey iron, showing the grain structure; and

FIG. 2 is a photomicrograph of a cross-section of the grey iron after treatment according to the invention.

In carrying out the process of this invention, a billet of grey iron is first cast into a desired volume and shape to fit conveniently into the die. The billet should be of such shape that it provides the desired metal flow characteristics.

A preferred apparatus to be used to apply the necessary pressure over a short period of time is described in copending application Ser. No. 410,365, filed Nov. 12, 1964, now US. Patent No. 3,404,555. In this apparatus, oppositely faced pistons are moved over a relatively short dis tance at extremely high speeds to apply great pressure upon impact with the billet for a short time.

Prior to introducing the billet into the die, the die is preferably heated to at least 250 F., but generally in the range of about 250 F. to 650 F. The billet is preheated to 1650 F.-l750 F. in a controlled inert atmosphere because the presence of oxygen will form a scale which may be undesirable. The time over which the billet is heated prior to introduction to the die is determined by its shape and volume, and may vary anywhere from 30 minutes to one hour.

The heated billet is then introduced into the die cavity. The die cavity is provided with air vents so that air will not be trapped within the cavity. The ventilation eliminates the trapped air which could otherwise prevent the complete filling of the die cavity.

In forming the billet into the desired configuration, the billet should undergo compression rather than tension. That is, the billet should be submitted to larger compressive than tensive forces. This condition results in successfully forming grey iron into a desired shape and upgrading the mechanical properties.

The desired configuration and properties of the finished part determines the pressures needed. In general, pressures between 30,000 to 90,000 p.s.i., preferably 40,000- 80,000 p.s.i., will be applied. The upper limit of the applied pressure is dependent upon the capability of the die to withstand fracture at that pressure. Higher pressures would provide improved results if sufliciently strong dies could be fabricated to withstand them. The rate of pressure application makes the material flow plastically, which in turn changes the random formation of the graphite flake shown in FIG. 1 to an oriented flake following a specific contour as shown in FIG. 2. This gives the grey iron its forging characteristics. Furthermore, the rate of pressure application will lead to a reduction in crosssection which will also upgrade the mechanical properties. The resulting product is thus capable of being substituted for higher grade materials.

At the present time, the only limitation to maximum reduction in cross section is the ability of the die material to withstand the high pressures, discussed above. The pressure is applied for a time in the range of from 1.5 to 15 milliseconds, or preferably from about 3 to 6 milliseconds. The temperature of the billet is in the range of about 1650 F. to 1750 F. A lubricant is used to protect the die from galling. Various commercial lubricants may be used, such as molybdenum disulfide, Kerns oil, etc. Kerns oil is preferred because of its ready availability and high flash point. In general, a graphite-based material will give the best results because it has the lowest friction coefficient.

The formed billet may then be removed from the die and further treated as desired. Hardening processes can be used to further improve the mechanical properties and annealing can be used for stress relief. The resulting prodnot obtained by using pressure forming of grey iron has significantly enhanced properties over untreated grey iron. The photos in FIGS. 1 and 2 indicate the difference between the grain structure of the grey iron prior to forming and subsequent to forming. In FIG. 1, the random orientation of the graphite is quite evident as contrasted to the relatively laminar orientation of the graphite in FIG. 2. The magnification of FIG. 1 is 100 times and the magnification of FIG. 2 is 200 times; a Nital etchant was used to show up the grain structure of the metal.

The density of the metal is increased and is a direct function of the reduction in volume from the cast billet to the formed part. Generally an increase in density is paralleled by an increase in hardness and tensile strength. The hardness and tensile strength are both a function of the reduction in cross section and volume reduction. In the example presented, the Brinell hardness increased from 210 to 325, which is an increase of 54.8% in Brinell hardness. The tensile strength increased from 37,000 p.s.i. to 48,000 p.s.i., which is an increase of 11,000 p.s.i. or 30%.

In order to demonstrate the excellent results obtained, a grey iron billetclass 40 (ASTMA 48)--was obtained; various portions were cut from the billet; the tensile strength of each portion was determined. The Brinell hardness of the billet was BHN207217. The billet, which weighed about 23 lbs. 3 02., was preheated at 175 F. for 60 minutes. It was then introduced into the die at a temperature of 1750 F. A Kerns oil lubricant was used on the die. An energy of 230,000 ft. lbs. was applied for about 4.5 milliseconds. The resulting object (formed as a flywheel) had a Brinell hardness of BHN- 255341.

The following table indicates the tensile strength of the billet and the formed object, from portions cut out from various indicated areas. The sample diameters were all 0250:0004 inch.

TABLE I Tensile strength Rim Web Section Circumferential Radial Direction Direction Before forming After forming To determine the efiects of centrifugal or dynamic forces on a flywheel prepared as described above, the flywheel was spun at 10,000 rpm. in a standard spinput for 5 minutes. At the completion of the test, the wheel was visually examined showing no indication of damage or growth.

The above process can be used with any grey or alloyed cast iron and the same improved results will be obtained.

By virtue of the method of this invention, where extremely high pressures are applied at elevated temperatures to grey iron in a closed die arrangement, not only is the metal shaped, but the process also upgrades the mechanical properties of the grey iron, providing tensile strengths in excess of those which are ordinarily met by grey iron. Moreover, the graphite flake is rearranged which improves the impact and wear properties of the metal. The presence of the graphite provides an internal lubricant, a further advantage for the use of grey iron.

As will be obvious to one skilled in the art, many modifications in the devices and method of fabricating them may be made without departing from the spirit and scope of the invention. Therefore, the only limitations upon that scope are those expressed in the claims which follow.

What is claimed is:

1. A method for shaping a grey iron element into a desired configuration having enhanced mechanical properties, which comprises:

compacting a billet of grey iron in a closed vented die at a pressure from about 30,000 to about 90,000 p.s.i. and a temperature from about 1650" to about 1750 F. for a short period of time.

2. A method for shaping a grey iron element into a desired configuration having enhanced physical properties, which comprises:

compacting a billet of grey iron in a closed vented die at a pressure from about 30,000 to about 90,000 p.s.i. and a temperature from about l650 to about 175091 for a period of time from about 3 to 6 milliseconds.

3. A method for shaping a grey iron element into a desired configuration having enhanced physical properties, which comprises:

heating a die to a temperature of at least 250 F.;

compacting a billet of grey iron in a closed vented die at a pressure from about 30,000 to about 90,000 p.s.i. and a temperature from about l650 to about 1750 F. for a period of time of the order of 5 milliseconds.

4. A method for shaping a grey iron element into a desired configuration having enhanced physical properties, which comprises:

compacting a billet of grey iron in a closed vented die lubricated with a lubricating oil, at a pressure from about 30,000 to about 90,000 p.s.i. and a temperature from about 1650 to about 1750 F. for a period of time of the order of 5 milliseconds.

5. A method for shaping a grey iron billet into a desired configuration, wherein the grain structure of the grey iron is modified to provide an object having enhanced tensile strength and hardness, which comprises:

introducing a grey iron billet into a vented die, compressing said billet by means of a punch, suitably fitted into said die, applying a pressure to said billet of about 30,000-90,000 p.s.i. at a temperature in the range of 16501750 F. for a time in the range of 4 to '6 milliseconds, withdrawing said punch from the formed object; and

allowing said formed object to cool.

6. A method for shaping a grey iron billet into a desired configuration, wherein the grain structure of the grey iron is modified to provide an object having enhanced tensile strength and hardness, which comprises:

introducing a grey iron billet into a vented lubricated die, compressing said billet by means of a punch, suitably fitted into said die, applying a pressure to said billet of from about 40,000 to about 80,000 p.s.i., for a time in the range of 3 to 6 milliseconds.

7. A method for shaping a grey iron billet heated to at least l650 F. into a desired configuration, wherein the grain-structure of the grey iron is modified to provide an object having enhanced tensile strength and hardness, which comprises:

introducing a grey iron billet into a vented die, compressing said billet by means of a punch, suitably fitted into said die, applying a pressure to said billet of about 30,000 to about 90,000 p.s.i. at a temperature in the range of about 1650 to about 1750 F.

for a time in the range of 3 to 6 milliseconds, withdrawing said punch from the formed object; and allowing said formed object to cool.

8. A method for shaping a grey iron billet into a desired configuration, wherein the grain structure of the grey iron is modified to provide an object having enhanced tensile strength and hardness, which comprises:

introducing a grey iron billet into a vented lubricated die, compressing said billet by means of a punch, suitably fitted into said die, applying a pressure to said billet of about 30,000 to about 90,000 p.s.i. at a temperature in the range of about l650 to about 1750 F. for a time in the range of 3 to 6 millisec- 5 6 ends, withdrawing said punch from the formed ob- 3,106,002 10/1963 Bauer 164-120 jects; and 3,228,073 1/19'66 Harrison 164-120 allowing said formed object to cool.

CHARLES W. LANHAM, Primary Examiner 5 G. P. CROSBY, Assistant Examiner US. Cl. X.R.

References Cited UNITED STATES PATENTS 2,241,270 5/1941 Nipper 148-138 148-138 

